The present invention is a method for altering or correcting the color gamut of an output device. It is a well-known phenomena in color printing to have, for example, the color blue (typically printed with maximum cyan and magenta and no yellow marking materials) rendered with a purplish hue or tint. Simply put, from a perceptual basis, many well-known colors are not be faithfully reproduced by equal combinations of primary printing colors. To accommodate this problem, a calibration operation is often performed for business graphics printing so that primary colors (red, green, blue cyan, magenta and yellow) are rendered in a manner to assure that the perceived color matches the color for which the name has a predefined color associated. It is known to employ multivariate linear regression to correct for nonpleasing primary colors, however, this is a global solution that may adversely affect other regions of the color space.
Accordingly, the present method was developed to correct gross errors for sensitive colors in documents, and employs a hue shifting operation to provide the improved color rendering.
In accordance with the present invention, there is provided a method for color correcting an image, the image being represented as a plurality of image signals in a first color space, comprising: converting an image signal representing a first device color space into a second device color space, wherein the second device color space is defined as a function of three dimensions and one of said dimensions represents hue; characterizing a hue shift function; applying the hue shift function to an image signal represented in said second device color space, to shift a first hue value of the image signal to a second hue value as determined by the hue shift function; and reconverting the image signal representing the second color space into a corrected image signal representing the first color space for rendition thereof.
In accordance with another aspect of the present invention, there is provided a method for warping a color space as a function of hue angle, comprising: mapping a first plurality of equidistant hue angles within a predefined range to a second plurality of non-linearly spaced, non-equidistant output hue angles; and recording the mapping therebetween receiving a first hue angle associated with an image signal, said first hue angle being one of the first plurality of hue angles; and producing, as a result of the mapping, a second hue angle for the image signal, wherein the second hue angle is one of the second plurality of hue angles.
In accordance with yet another aspect of the present invention, there is provided a method for color correcting an image, the image being represented as a plurality of image signals in a device color space, comprising: characterizing a mapping function, the mapping function mapping at least one input thereto to an alternative color wherein the alternative color is determined by a shift in a single component of an alternative color space; and applying the mapping function to each of the plurality of image signals to produce a plurality of modified image signals for rendering by a printing system.
One aspect of the invention is based on the discovery of a new technique for warping or otherwise shifting the hue within a color space to improve the color rendition of a printing system. More specifically, the technique accomplishes a simplified, one-time, warping of the color space to provide a vivid color output capability that may be selected by a user of a printing system. Upon invoking the vivid color mode, the printer employs the vivid color output capability. A machine implementing the invention can include any printing device capable of rendering output colors using a combination of two or more marking material colors.